Laser light has been used in various fields due to its high energy density and high coherence property, and a laser application technology has been used as an inspection tool, a preventive maintenance tool, and a repairing tool in the field of maintenance of a rector in a nuclear power plant. Examples of the laser application technology include laser deposit welding for repairing cracks in a structure and laser peening for preventing stress corrosion crack of structure in the reactor vessel.
The laser peening is a preventive maintenance technology using Nd:YAG (neodymium:yttrium-aluminum-garnet) laser. Laser light is collected using a lens and the like and irradiated onto the surface of a material to generate a plasma, and a compressive residual stress is applied to the material by a plasma shock wave. The stress corrosion crack occurs when the following three conditions are met: material condition, environment condition, and stress condition. However, by removing a stress condition (tensile residual stress) using the laser peening, occurrence of the stress corrosion crack in the vicinity of a welding part can be prevented.
In the case where the laser peening is applied to a structure in the reactor, a laser oscillator is disposed on an operation floor to transmit laser light to a laser irradiation head installed in the bottom part of the reactor. Examples of a means for transmitting the laser light include one that transmits the light using a mirror provided inside a hollow light guide tube and one that transmits the light using an optical fiber.
In the case of the mirror transmission system, pulse energy that can be transmitted to a machining target is sufficiently large. However, as the transmission distance is increased, the size of the apparatus is increased correspondingly. In the case where the laser light is transmitted from the operation floor to the bottom part of the reactor, the transmission distance roughly reaches up to 40 m, which correspondingly increases the size of the apparatus for mirror transmission and requires taking the influence due to vibration of a light guide path into consideration. On the other hand, in the case of the optical fiber transmission system, simple connection of the laser oscillator and laser irradiation head by an optical fiber allows laser light transmission. Thus, the size of the apparatus need not be made large even when the transmission distance is increased, so that the influence due to vibration of a light guide path can be ignored. However, there may be a case where the optical fiber is damaged by the laser light, so that a system for monitoring the damage of the optical fiber is required. Further, the energy density of the laser that can be transmitted using the optical fiber transmission system is smaller than that in case of the mirror transmission system.
As another means for applying the laser peening to a structure in the reactor, there is known an apparatus that includes, in the reactor, a watertight vessel housing a laser oscillator and uses a mirror to transmit the laser light from the laser oscillator to a laser irradiation head. Such an apparatus is disclosed in Jpn. Pat. Appln. Laid-Open Publication No. 2005-227218, the entire content of which is incorporated herein by reference. With this configuration, a simple apparatus configuration can be obtained by reducing the laser transmission distance. Further, the energy level of the laser that can be transmitted is high, so that machining can be done with low irradiation density, thereby reducing machining time. Further, a positioning mechanism is simplified to achieve positioning at short times.
A laser oscillator used in such a laser peening apparatus is influenced in its performance by the temperature of environment in use. In general, the temperature range is specified in the laser oscillator, and when the temperature of use environment is too high or too low with respect to the specified temperature range, the performance deteriorates. Thus, generally, cooling water is used to cool the laser oscillator so as to prevent the laser oscillator from becoming a state of higher temperature due to heating of the oscillator itself. In general, a cooling water supplying apparatus is mounted on a power supply apparatus for the laser oscillator to cool the laser oscillator by supplying the cooling water.
However, in the case of the abovementioned apparatus, when the cooling water is supplied to the laser oscillator from the power supply apparatus on the operation floor, the temperature of the cooling water flowing in a supplying path is influenced by reactor water. It is possible to adjust the temperature of the cooling water with the influence due to the reactor water taken into consideration if the distance from the water surface to the laser oscillator is small. However, the temperature of the reactor water may change according to the depth and, therefore, the useful range of the above method is within about 5 m depth. The reactor pressure vessel has a height of 20 m or more, so 5 m is not sufficient.